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Choosing Soil for Bonsai Trees

Introduction

PART 1

 

PART 2

 

PART 3

 

PART 4

GLOSSARY OF TERMS

 

 

INTRODUCTION
Nothing in the whole sphere of bonsai cultivation causes more angst and controversy than soil mixes. Getting your soil right is the foundation of creating great bonsai. Roots are to bonsai what an engine is to a car. Just as a car without an engine is a useless tin box so a bonsai tree without roots is a useless bit of wood. Just as a teenager with his first car spends all his time making it look good, with little regard to the mechanical integrity or reliability, so most folk new to bonsai concentrate their efforts on how their tree looks rather than it’s health and vitality.

In order to enjoy success with bonsai our overriding concern should be for the health and well being of our plants. Styling of a tree is always secondary to this requirement. A healthy tree can have shaping deficiencies rectified in time. The bonsai world is littered with the carcases of spectacular dead trees that were ‘styled’ into oblivion. A live tree can have its problems addressed over any number of years but it’s all over for a dead tree. It is very important to understand this simple principle in order to enjoy success with bonsai trees.

The beauty of a bonsai tree comes from within as a result of exemplary horticultural practice and respect for the tree. We need to respect the fact that trees live by a different time clock to us. An 80 year old tree is just getting started but for most of us it’s almost over at that age.

Selecting the wrong soil for a bonsai can have severe consequences. Alternatively selecting the correct soil and coupling this with good horticultural practice can absolutely transform a tree and our enjoyment of that tree. I have found that nothing compares to the joy of working with strong vibrant healthy plants that respond positively to our guiding hand.

Selecting the correct soil for bonsai appears confusing and indeed many folk have some clever, normally misguided, idea about some ‘magic’ formula. Much like a bunch of Victorian quack doctors boiling up pots of weeds and sticks in order to cure diseases about which they knew nothing. However by understanding a few simple basic principles it is very easy to select the perfect soil for your bonsai. Cultivating bonsai is the same as cultivating any other plant because plants have simple needs that are easily supplied, once we know what they are of course!

Since the first day I began keeping bonsai I have had a fanatical fascination with the relationship between a plant and its soil. Understanding how a tree interacts with its growing media, and applying that knowledge wisely is the secret to creating beautiful emotive bonsai trees that touch our soul.

What follows is a synopsis of what I have learned over more than 20 years of relentless study. I know there are a lot of words here but master this subject and you will be successful at cultivating bonsai. I hope it turns a light on for you as it has for me.
Graham Potter.
January 2011.

PART 1

HOW BONSAI TREES GROW
Before we can decide what soil to plant our bonsai tree into it’s important to know exactly what our tree needs from that soil to support itself and grow happily for the immediate future. The following gets a bit complicated but once you get these basics under your belt choosing soil for bonsai cultivation will be very simple.

 

THE RECIPE FOR GROWTH
Much like other living organisms plants need an energy source, unlike animals that eat other living things plants make their own energy from simple freely available elements.

The process of photosynthesis is the basis of all life on earth. Large animals eat smaller animals that eat plants that produce energy by photosynthesis.

Photosynthesis is the process that plants use to create carbohydrates to fuel their growth. The ingredients are simple…

carbon dioxide + water + sunlight = carbohydrate + oxygen + water

Plants absorb light primarily using the pigment chlorophyll, the green element of leaves, and use that light energy to fuel the chemical reaction outlined above.

Minerals are used by plants to produce green chlorophyll that absorbs light energy for photosynthesis. Production of chlorophyll must be continuous, since it loses its efficiency quickly. A plant deficient in iron or magnesium especially, turns yellow (chloritic) and loses much of it’s ability to photosynthesize, hence the need for regular fertilizing.

Carbohydrates (simple sugars) are used as the energy source to fuel cell division and thus growth of a plant. The process of releasing energy from carbohydrates is called respiration. And here comes the key point.

Carbohydrates+oxygen+water = Energy+carbon dioxide+water

Without a sufficient oxygen supply carbohydrate breakdown is incomplete and the formula reads a little differently.

Carbohydrates+oxygen+water = Energy+carbon dioxide+water+ethanol (alcohol)

And there you have the whole crux of why the correct soil mixture is so important for bonsai cultivation. Insufficient air circulation within the soil means a lack of oxygen and as a result respiration (for cell division for root growth) is incomplete and a byproduct of this is alcohol and that’s poisonous to plants, it causes root rot.

 

THE CIRCLE OF LIFE - BONSAI
The flow of water, minerals and carbohydrates in a tree goes both ways. Water and minerals go up (in the xylem), from the roots, and carbohydrates from the leaves come down (in the phloem), from the leaves. Plants do not take ‘food’ from the soil, rather raw materials that are turned into food within the leaves.

The process begins by water (which contains dissolved minerals and chemical compounds) soaking into the cells of the roots through a porous membrane. The solution passes from cell to cell until it reaches the leaves. Using sunlight as the energy source the chlorophyll within the leaf uses carbon (from carbon dioxide in the air, absorbed through the leaf) and water (from the roots) to create starch or carbohydrates which are what the plant needs to grow. The dissolved minerals within the water coming up the tree are used for the creation and maintenance of chlorophyll, not directly for growth. The by product of photosynthesis is water and oxygen which is expired through the leaf.

Once the plant has created carbohydrates in it’s leaves this is transported (using water) to all parts of the plant including going down into the roots. These carbohydrates are used to fuel cell division for growth by a process known as respiration. Hormones within the plant determine what type of cells are formed. The plant also has the capacity to store excess carbohydrates.

 

ROOTS NEED TO BREATHE
Respiration is the process by which a plant burns carbohydrates to fuel cell division for growth. In the root zone complete respiration uses oxygen and water to release the energy within starches and produces byproducts of water and carbon dioxide. If there is insufficient oxygen present within the growing media anaerobic respiration takes place and the byproducts are water, carbon dioxide and ethanol (alcohol). This leads to poor root growth at best and root rot at worst. Root rot is quite literally alcohol poisoning caused by a lack of oxygen. If this happens the whole chain of events will be broken and growth will ultimately cease.

In order to maintain good plant health and growth it’s important to ensure good horticultural practice. This includes using soil suitable for the purpose and plant in question.

 

TO SUM UP
Strong roots ensure a well fueled photosynthetic process. This produces lots of fuel for growth that produces strong roots that produces strong leaf growth, that produces stronger roots and so it goes around. Master this equation and your bonsai trees will become incredibly strong, will develop quickly and will be pest and disease free.

 

PART 2

BONSAI POTS – A UNIQUE CHALLENGE
Cultivating plants in pots is much more difficult than growing plants in open ground. Cultivating plants in shallow pots is more difficult still and cultivating very old plants in very small volumes of soil is really pushing it to the edge.

A bonsai pot is a very challenging environment in which to grow a tree, indeed it’s one of the most challenging things to achieve in horticulture. Getting it just right is a very highly skilled job that can only be perfected over decades of experience and meticulous observation and experimentation.

Typically a bonsai pot holds a fairly small volume of soil. This small amount of soil has a relatively large surface area that leaves it very susceptible to moisture evaporation. It is also subject to dramatic temperature changes in a very short period. On a warm sunny day the soil can see a 40° temperature swing. In winter if the soil freezes the significant pressure caused by swelling in a small enclosed space can severely damage roots. In wet weather it’s very easy for a bonsai pot to become waterlogged.

These are just a few of the problems that can occur!

 

WATER – THE BASICS
In light of the above it’s obvious that plants need water for a variety of purposes including transportation and utilization of nutrients. Most of the water in a plant will be absorbed through the roots. However, too much of a good thing is bad. With a few exceptions most plants will not stand having their roots submerged in water for long because the lack of oxygen will cause root death. Simply put most plants absorb humidity from the soil not liquid water. Therefore a soil that is damp, but not wet is required for optimum health and growth. Following watering we need the excess water to drain from the pot, leaving the soil moist but not soaking.

 

UNDERSTANDING DRAINAGE OF BONSAI SOIL
Understanding drainage is of paramount importance in the quest to find the perfect growing medium for bonsai. Drainage is simply the process by which excess water escapes from the growing medium but there are a number of factors that bear upon the efficiency with which this happens.

After watering it is important that liquid water is removed from the soil as quickly as possible for the health of the plant (to get oxygen back to those roots). We are almost entirely dependent upon gravity to achieve this. Going back to school physics we know that water has a surface tension that causes it to cling to a surface. Throw water at a vertical smooth surface and most of it will fall away but some clings on because it’s surface tension is stronger than gravity pulling at it.

Obviously soil consists of lots of particles with small spaces between them. Put simply the smaller and more closely packed the particles are, the less drainage can be achieved because the water has a lot of surface to hold onto, thus overcoming the force of gravity trying to pull the water down. Conversely the larger these pore spaces the faster water will pass from the soil.

Getting the balance of drainage just right is one of the key factors of selecting the perfect soil or growing media for bonsai.

Unfortunately it’s not possible to supply a simple answer to how much drainage is enough because there are so many variables including the fact that different types of plant have differing water needs. When we understand the principles involved we can make a very good educated guess, which will provide a good starting point. Over the years we can adjust a plants growing medium to provide the perfect solution.

 

POROSITY OF SOIL
Just as we live in the space inside our houses, plant roots live in the spaces between solid soil particles. The term for the amount of air held within a growing medium is – Air Filled Porosity (AFP). Soil particles also soak up an amount of water if they are porous. Here we are more concerned with the spaces between the soil particles.

In general the smallest spaces, micro pores, contain water which, because of their minute size cannot be accessed by even the finest root hairs. The middle size pores, mesopores, contain water available to plants and air moves into these as the plant removes the water. Pores greater than about 0.1mm diameter, called macro pores, will drain to allow free air circulation within a short period after watering. Ideally there should be sufficient mesopores to ensure good retention of water but sufficient macro pores to allow free drainage, gaseous exchange and thorough root exploration.

The key to selecting a perfect growing medium is in maintaining a high proportion of air-filled pores without restricting water supply.


OTHER DRAINAGE FACTORS
Having looked briefly at the subject of drainage it’s now necessary to add another significant factor of which I have never seen a mention in bonsai literature. However it’s such a powerful and significant force, and is responsible for the death of a million bonsai trees, that it’s being overlooked within the tome of published literature really is a crime. I am talking about capillary action and it really throws a spanner in the works.

Capillary action, or capillarity, is a phenomenon where liquid spontaneously rises in a narrow space such as a thin tube, or in porous materials. The thinner the capillaries (tubes) within a material the higher water will rise due to this force. In the right conditions capillary action overcomes gravity and water goes up hill. As a result the drainage in our soil will cease altogether.

Capillary action is of relatively little consequence in summer when a plants roots are constantly stripping moisture from the soil.

However, in winter that is not the case. I live in the U.K where winters can be wet and temperatures are mild. Winter temperatures vary between -4 to +6-10 Celsius, snow is a rarity and frost rarely persists for more than a couple of days. Bear that in mind as we look at a couple of scenarios of bonsai planted in pots and overwintering outside.

Firstly, let’s consider a tree growing in a nice deep pot in a well chosen quality soil mix. As water passes into the pot it quickly permeates down through the soil, creating a slight vacuum or draw behind it, air is drawn into the soil. A carefully graded soil allows the water to drain to the bottom of the pot where it builds up a little before escaping from the pots drainage holes. Capillary action is weak because the soil has good drainage characteristics, the capillaries formed by the soil grains are large. So a small concentration of water exists at the bottom of the pot but not enough to really trouble the roots. Because the air content of the soil is good, freezing has little detrimental effect as expansion is easily absorbed within the soil yet air circulation is maintained.

Secondly, let’s consider a tree growing in a shallow pot in an old fashioned soil mix like John Innes No2 and grit. Grit (crushed stone) particles are large, hard, cold and not absorbent. John Innes is a soil based mix with a high proportion of sand and clay. The grit within this mix settles into a rigid framework. Over summer during constant watering the soil particles are washed into the pore spaces between the grit. The clay within the soil is broken down and, because the particles are so fine they end up right at the bottom of the pot. Therefore a layer of ultra fine particles or silt form on the base of the pot. The soil above has some drainage and draws air in as water runs out. In summer when roots are stripping water from the soil all works well enough. However in winter because of the very fine composition of the lower layers of soil, capillary action holds a deep layer of liquid at anything up to three quarters of the depth of the pot. In effect the plant will be standing in a puddle. Constant freezing and thawing will destroy the roots of all but the most resilient plants. The result will be very slow poor growth in spring and ultimately very slow development of the bonsai.

So, in light of the above it’s plain to see that the growing medium has a very different role to perform in the summer than in the winter. The medium we choose is influenced by many factors including…

The shape/depth of the container.
The individual needs of the plant species.
The level of maturity of the bonsai.
The type of growth required (mature bonsai or raw material)
The environmental situation including exposure to sun and wind.
How often you can water.
The fertilizer you use.
Winter conditions.
The pH requirements of the plant.
The pH of the water you have.
Temperature extremes both summer and winter.

 

PHYSICAL AND MECHANICAL REQUIREMENTS OF BONSAI SOIL
Bonsai trees are imprisoned in very small pots with their small volumes of soil. In practice this does not present a problem to the health of the plant so long as we provide everything that plant needs within that small space. Given correct horticultural practice a bonsai can be every bit as healthy and vigorous as plants growing in open ground. What we need is a very small but powerful little root system that is working at optimum efficiency. Room for error is all but non-existent.

As we have seen, our soil needs to contain water and air as well as holding nutrients in order for our plants to grow. Because bonsai remain in their pots in the same soil, often for years, the soil we choose needs to have sufficient mechanical integrity to maintain optimum growing conditions. Choosing a good open soil medium is fairly easy but, if after a couple of frosts or extensive drying in summer, it turns to powder then it’s valuable properties will be lost and our little tree will suffer.

The issue centres around the mechanical strength of a soil component. Constant expansion and contraction due to temperature and moisture fluctuations can take their toll on softer materials. As water freezes it’s volume increases by 9%. A product like Moler retains up to 108% (Westinghouse) of it’s own volume of water. So a saturated soil particle will expand considerably upon freezing. If that particle does not have sufficient mechanical integrity or elasticity to absorb that expansion it will rupture. This can happen over and over until your carefully sieved soil particle becomes dust and so your soil loses it’s all important pore spaces.

This effect is thankfully not as serious as it could be due to a process called. Cryosuction. This is the process in freezing soils whereby water migrates through soil pores to the freezing zone via capillary action. Water tends to move out of the soil particle itself and into the spaces around the grain where it then freezes.

Products like crushed stone and other aggregates suffer very little ill effect from freezing. Also organic materials like bark are rarely affected. However more fragile products like Akadama can be very quickly destroyed by a hard frost, especially if this is preceded by extensive rainfall or watering.

The flip side of this issue involves heat and water expansion. It’s common (even in the U.K) for summer pot temperatures to reach 30-40 Celsius where a pot is exposed to full sun. Heat expands most materials and soil is no exception. Our watering frequency increases in summer and this not only rapidly cools the soil but can also cause further expansion as it soaks into the particles.

Also of concern is the fact that continual water passage through the soil can literally dissolve soil granules. Whilst most of the dissolved solids will simply drain out of the base of the pot a large proportion of this incredibly fine silt will build up in the pot. This is why using something like natural clay particles, chalk or soft stone can present a real problem. Also bear in mind a wide differentiation between the pH of your water and the pH of your soil can lead to chemical erosion of soil particles.

All together soil is subject to some very powerful thermal and chemical forces that can easily destroy its integrity.


PART 3

FACTORS CONCERNING NUTRIENTS

CATION EXCHANGE CAPACITY (C.E.C)
For bonsai, growing in very small volumes of soil, the ability of the growing medium to retain nutrients is very important. A soil’s ability to hold onto nutrients is known as its cation exchange capacity. Plant nutrients in soil are always in the form of chemical compounds and these can easily be washed from a poorly structured soil mix.

Clay particles carry a negative electrical charge that means they attract positively charged cations within the soil. These include the nutrients potassium, ammonia, magnesium and calcium as well as hydrogen and aluminium ions. These are held in an exchangeable way so that they remain available to plants but are prevented from being leached out of the soil by water.

In order for bonsai to grow strongly soil needs a high cation exchange capacity so it will hold lots of nutrient. Clay is the primary material we can use to achieve this, however clay has a tendency to break down quickly and it’s minute particles will turn a free-draining bonsai soil into concrete in a very short time. Thankfully there are several hard clay based products we can use that will work very efficiently for us. These include Japanese Akadama and Horticultural Moler. Crushed aggregate products like flint grit or granite have almost zero cation exchange capacity.

Bonsai, growing in a soil with a good C.E.C look healthy with strong, uniform leaf growth. Over the growing period they perform well putting out strong even growth across their branches without weak points. These trees are resilient and strong and are all but impervious to pest attack. Deciduous trees hold their leaves well into the autumn and display good autumn colours. Over winter the trees are resistant to cold and drying winds and will not suffer die back of fine growth. In spring they grow away quickly and strongly.

 

NUTRIENT REQUIREMENTS OF BONSAI SOILS
Any vegetable gardener will tell you it’s important to get lots of good quality nutrient-bearing organic matter into your soil over winter to nourish the following seasons crop. That’s absolutely true but, by some strange process, the thought has leached into the tome of bonsai knowledge where it seems to be as firmly entrenched as Japanese knotweed. The term is known as “base dressing”, the process of adding nutrient to the soil prior to planting.

Leading on from the above there is also a thought that a bonsai soil should be ‘rich’ or in some way contain special nutrients to ‘nourish’ the tree. Much like our vegetable gardener many folk spend ludicrous amounts of time preparing all sorts of magical organic ingredients including leaf mould, special composts and an endless list of alchemical potions.

Under close scrutiny, in the cold light of day most of the old wives tales about bonsai soil are just that… the incongruous thoughts of the ill-informed. Let me explain.

Bonsai grow in implausibly small amounts of soil. A healthy growing tree will strip all the nutrient from its soil within two weeks of active growth.

Most organic matter, including leaf mould, is very low in nutrient value to begin with. Just a few days in a bonsai pot at a warm temperature and with regular watering and its value is all but zero.

Our vegetable gardener has to add nutrient-bearing materials to his soil every year, normally large volumes of animal manure. Bonsai trees stay in their pots anywhere from two to twenty years. Any nutrient added will be gone in a few weeks, then what?

High nutrient content of bonsai soil will tend to cause problems to recently re-potted trees. If you use a nutrient-dense compost at re-potting time you will severely impede the trees ability to recover and resume normal growth.

Introducing organic materials will always bring a bewildering array of fungi, bacteria, micro-organisms, enzymes and pathogens along with it. Whilst some of these are very good the potential is that some will be very bad, and immediately after re-potting, a tree can be vulnerable to attack/infection. It’s just not worth the risk.

 

HUMUS
Humus is important in bonsai soil. However composted organic materials are not humus. Humus is what is left AFTER organic material has completely decayed. It’s the black substance that coats soil particles that typically turn topsoil black. Humus has a high cation exchange capacity. So we see our gardener, mixing compost into his soil will, in time, increase the nutrient holding capacity of his soil. The organic material is a means to an end but, not an end in itself. Any inherent nutrient added is a short-lived bonus.

Mixing too much organic material into bonsai soil will lead to an excess of moisture and lowered oxygen content. This will slow the breakdown of the organic matter and will seriously impair the tree’s ability to grow roots and absorb nutrients. As often seems to be the case what we were trying to achieve has been made more difficult by the very thing we did to achieve it.

So, what requirements do we need to fulfill in relation to the nutrient content of bonsai soil? The answer couldn’t be simpler. Bonsai soil does not need to have inherent nutrient value. After re-potting a tree will recover much faster in a nutrient free, clean aerated growing medium. As long as we choose ingredients for our soil with a high cation exchange capacity the entire nutrient requirement of the tree will be taken care of by our fertilizing regimen.

In order to increase the cation exchange capacity of our soil mix we need to increase its humus content. This will naturally happen if we use a good quality organic fertilizer like Green Dream. Once the nutrients have been used up soil bacteria will quickly break down the residues to form humus. This process is less efficient when using traditional fertilizer cakes that sit on top of the soil. If your own preference is for chemical based fertilizers such as Crystal Specialist Fertilizer you can still increase the humus content of the growing media by regular use of a humate product like Root Safe.

To sum up… Bonsai soil does not need inherent nutrient content. All of a bonsai’s nutrient requirement is taken care of by regular use of fertilizers. In order for fertilizers to be effective the growing media needs to have an ability to grab hold of nutrient compounds long enough for the plants roots to absorb them. This happens at an atomic level and is based on the electrical charge of soil atoms and is commonly known as the soils cation exchange capacity (C.E.C).


SOIL FLORA & FAUNA
Soil not only supports our plants but it provides a home for countless tiny life forms including a bewildering array of fungi, bacteria and insect life. A single gram of garden soil can contain a million fungi such as yeasts and moulds. Whilst some of these are detrimental to our trees many are crucial to their wellbeing.

In a balanced soil, plants grow in an active and vibrant environment. The mineral content of the soil and its physical structure are important for their well-being, but it is the life in the earth that powers its cycles and provides its fertility. Without the activities of soil organisms, organic material would accumulate and litter the soil surface, and there would be no food for plants. The soil biota includes:

Megafauna: size range 20 mm upwards, e.g moles, rabits and rodents.
Macrofauna: size range 2–20 mm, e.g. woodlice, earthworms and beetles
Mesofauna: size range 100 micrometer-2 mm, e.g. mites and springtails
Microfauna and Microflora: size range 1-100 micrometres, e.g. Yeasts, bacteria, fungi, protozoa, roundworms and rotifers.

Of these, bacteria and fungi play key roles in maintaining a healthy soil. They act as decomposers that break down organic materials (such as fertilizers) to produce detritus and other breakdown products. Soil detritivores, like earthworms, ingest detritus and decompose it. Saprotrophs, well represented by fungi and bacteria, extract soluble nutrients from delitro which in its turn can be absorbed by plants.

Plants growing in sterile soil environments tend to have poor growth and development as well as having increased susceptibility to pests and disease. Many ingredients of modern bonsai soil, due to their manufacturing process, are sterile. Contrary to popular belief this is a bad thing.

This subject is far too involved to be covered here but suffice to say that most of the beneficial organisms of soil will either be in the existing soil attached to the tree’s roots or they will build up over time with proper soil management. All we have to do is make sure our soil presents a good air-filled moist environment for soil’s bewildering array of flora and fauna to thrive; as a result our bonsai will thrive.


SOIL pH
Life in soil is greatly influenced by the soil pH. The pH scale is a means of expressing the degree of acidity or alkalinity. The scale runs from 0-14, with 7 being neutral, 0 being extremely acid and 14 being extremely alkaline. The scale is logarithmic, so pH6 is 10 times more acidic than pH7. pH5 is 100 times more acidic than pH7. For ideal growing conditions most plants require a soil pH of about 6.5 that is slightly acidic. At this point most of the plant nutrients are available for uptake by the roots. Alkaline conditions are usually caused by the presence of calcium (lime).

Calcicoles are plants that are adapted to living in very alkaline soils.

Calcifuges are lime hating plants that are adapted to acidic conditions, even small amounts of lime in their soil will cause serious problems with their growth.

The pH of a soil directly affects the availability of many nutrients to a plant. For instance a very acidic soil will restrict the availability of nitrogen. Therefore getting the pH of your growing medium just right, and closely matched to your plants needs, is important.

Many factors conspire to change the pH of a soil over time including, the pH of our water and the type of fertilizer we use. Adjusting pH is a very difficult thing to get just right and for bonsai is not recommended. The best plan of action is to determine what pH your plant needs and select a soil medium that is correct at the outset. Over time this may change slightly but regular re-potting will keep this change in check.

Plants that show continued reduced growth performance over several seasons, impaired flowering, foliage form or colour and susceptibility to disease may well be suffering in a soil with an inappropriate pH. Soil test kits are readily available and provide a reliable way to determine the pH of a soil. A quick internet search of good horticultural site and plant indices will provide the appropriate requirements of most plants.

In practice it’s not necessary to have a deep understanding of a plants pH requirements. Any good retailer can provide soils with an appropriate pH for most commonly cultivated plants. However beware of cheap “Basic” mixes from some suppliers. These are often based on spent mushroom compost or cheap commercially composted garden waste which are great for garden use but NOT for bonsai cultivation. Their wildly fluctuating pH can cause havoc with some plants.

If in doubt about the needs of a particular plant start with a good quality professionally produced bonsai soil mix with a pH around 6.5. Ericas such as azaleas and heathers need a mix around pH5.5. Over the following couple of seasons observe carefully the quality of growth and health of the tree.

 

PART 4

PREPARATION OF BONSAI SOIL
All bonsai soils are a compromise between what is ideal and what is possible. As a result the ideal medium will most likely be a blend of different components that bring together unique qualities. Careful preparation of those components is very important and will make the difference between success and failure.

Much has been written about the merits of sieving bonsai soils. Most of this is concerned with the eradication of dust. Removing very fine dust is helpful, particularly when using products like Akadama. Dust will settle at the bottom of a pot and can hinder drainage. However as always that’s not as simple as it sounds.

Very fine dust particles found in various potting medium such as Akadama, pumice and their ilk are produced by the abrasive action of particles rubbing together. This produces a very fine powder that adheres to the particle. No amount of sieving can remove this, in fact it makes it worse. Simply watering a newly potted tree until water floods out of the pot and runs clear will remove all of this unwanted material.

Where organic materials are concerned, for instance bark, the ‘dust’ or more correctly ‘fines’ can be an extremely beneficial addition to our mixes. Unlike the dust produced from hard materials that congeal in the bottom of the pot, organic fines can remain relatively mobile within the mix and will very rarely solidify. They are very good at reducing evaporation rates and improving moisture-holding capacity. Organic fines are quickly broken down by bacteria, forming beneficial CEC-enhancing humus, as explained earlier. So, some organic fines we can use to our advantage.

By far the most important aspect of sieving is that of grading our materials. The old fashioned practice of mixing grit with a loam and peat based soil mix is, upon examination ludicrous. Grit settles in a pot to form a fairly rigid framework of spaces and each particle rests upon those below. Mixing in something like a loam based soil is just crazy because constant watering washes this into those, all important, spaces almost completely excluding air from the mix. Over time the compaction continues as heavy sand grains sink lower in the pot. Fairly quickly all of the attributes of a good growing medium are lost and our tree begins to suffer. It’s a testament to the resilience of trees that they live in this muck at all.

One of the secrets of producing a good bonsai growing medium is in sizing all the particles within a pot carefully. Finer grain soils decrease drainage and increase moisture-holding capacity and larger grain size does the reverse. Using a Bonsai soil sieve to grade particle sizes uniformly is important. In order to adjust the moisture holding capacity further carefully graded organic material can be used.

 

COMMONLY AVAILABLE BONSAI SOIL COMPONENTS
There are a great many products available to choose for growing your bonsai trees. These are best broken down into ‘Straight’ materials and prepared ‘Soil Mixes’. Straights include materials like Akadama, Pumice and Bark. Mixes consist of various proportions of products mixed together to produce a soil suitable for a specific purpose and will incorporate advantages of many of its ingredients.

 

CHARACTERISTICS OF SOME POPULAR SOIL COMPONENTS
What follows is our opinion of most commonly used soil mixes and components. All of the products detailed have been extensively tested on our nursery over a 20-year period. We are based in the east of England and so this information should be viewed in the light of the climate we have here (mild).


AKADAMA
A naturally occurring volcanic sub-soil from Japan. Produced commercially for bonsai cultivation and other specialized horticultural disciplines.
Available in various grades (particle sizes).
Has a high C.E.C and neutral pH with excellent drainage characteristics and water absorbency if carefully graded. Has a very attractive brown colour.
There are two major drawbacks with this material, the first being cost. Wage costs in Japan are high and it has to be transported half way around the world. The second, and most significant, failing is that the product is not frost proof, in a climate like that of the U.K. Akadama can be reduced to dust after just a couple of severe winters and this will render it useless for cultivating bonsai.

 

KANUMA
A remarkable light, soft, volcanic, pumice-like, material from Japan.
A very acidic material suitable for growing ericas. Typically pH 5.5. The soft airy structure means roots actually penetrate the particles. As a result accurate grading is not necessary.
This is about the lightest weight growing material available but is has a tendency to float on water which can be annoying. Fairly expensive because it has to travel half way around the world. There are much cheaper and long lived materials available.

 

KIRYU
A hard heavy Japanese type of dense pumice that is moderately absorbent and with a neutral pH. Hard frost-proof structure means it is long lived and suitable for bonsai such as pines that are rarely re-potted. Low C.E.C., heavy and expensive. More expensive graded forms have much less dust and fine particles so are normally better value. There are better cheaper alternatives available today.

 

EZO GRIT
A Japanese volcanic pumice with a very attractive buff colour. Good moisture-holding capacity and frost proof so, long lived. Low C.E.C. Moderately heavy and very expensive due to transport and import costs. European produced pumice is every bit as good and much cheaper.

 

FUJI GRIT/SAND
Jet black crushed volcanic lava ash. Very attractive colour. Moderately absorbent but very heavy. It’s hard to understand exactly why we bring this product half way around the world.

 

HORTICULTURAL GRIT
Available in various forms from fine sand to gravel. This is basically crushed stone, normally quartzite or granite. Generally used for digging into heavy clay soils and opening up the soil in raised beds.
Probably the heaviest material you could find to fill a bonsai pot with. Grit is heavy, cold and tends to offer very little drainage. It has no absorbency and no nutrient holding capacity. Over time with constant watering smaller particles fill it’s pore space and the root ball will congeal into a solid sopping wet mass. Probably the cheapest material you will find to fill a bonsai pot and in our experience absolutely the worst potting medium you can buy. Often it’s mixed with other products like Akadama to make them go further. We think that’s seriously false economy. Use for dressing bench tops and filling pot standing beds or humidity trays only.

 

JOHN INNES MIXES
John Innes mixes are popular in the U.K. These are carefully formulated loam or peat based compost mixes designed for general horticultural use. They work well in conventional flowerpot style containers. Their popularity for bonsai use probably stems from a time when little else was available here. However bear in mind that most commercially produced pot grown plants are designed for planting out within 1-3 years of production. John Innes composts are not designed for long term containment of plants. Because of the composition of these products (peat/sand/loam) their drainage in a bonsai pot will be extremely limited. Whilst it is possible to achieve reasonable success using a John Innes mix and grit, which is an immensely heavy mix, the risks are hardly worth taking. Modern bonsai growing mediums are so easy to use, light and airy, and guarantee success, that it seems pointless to use this old problematic product.

 

PERLITE
Perlite is manufactured from a type of volcanic glass that is heated to 850-900°C. It expands rapidly to 7-16 times it’s original volume to form an ultra light-weight, hard, but very absorbent, material. It is popular in horticultural use for improving drainage and reducing weight in conventional composts where it is mixed at 10% by volume. For bonsai use perlite is useful for helping to reduce weight of large pots especially large growing containers and boxes used for establishing collected trees. It can be mixed with other materials intended for use in bonsai pots. However its lack of nutrient-holding capacity and bright white colour mean it’s not ideal. Importantly, bear in mind that perlite is often lighter than water and so tends to float. Over time it seems to come to the surface of a soil mix. When very dry the wind will blow it away. Overall a very good material that’s lightweight and cheap but it does have some drawbacks.

 

MULTI PURPOSE COMPOST
These are commonly available at garden centers and hardware stores everywhere. Generally available as peat based or peat free formulations. Because of their ultra fine fibrous nature they are completely unsuitable for use in bonsai containers and should be avoided at all cost!

 

PEAT
Many old books and magazine articles recommend the use of peat. The usual advice is to sieve and grade thoroughly using only the small nuggets or chips left at the end. This is immensely time consuming and difficult considering that peat is normally shipped in a slightly damp highly compressed form. By the time you have sieved modern peat products you will have less than 10% to use. All of those little pieces will end up, after a few weeks, in exactly the same condition as the material you have sieved out. Peat is simply old, densely packed composted sphagnum moss. It’s primary feature is that it holds a lot of water, it is after all the material responsible for ‘peat bogs’ and their unique wet environment. Our recommendation is to leave it where it is, supporting a very important eco-system.

 

VERMICULITE
A heat expanded mica clay that is popular in commercial horticulture owing to the fact it has a very high cation exchange capacity. It is best avoided in bonsai cultivation due to it’s tendency to break down and impede drainage. Also fine dust from vermiculite has proven to be harmful to health

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PEA SHINGLE/AQUARIUM GRAVEL
The use of shingle has been popular in bonsai cultivation for a long time. It is normally employed to improve drainage. It has no moisture or nutrient holding capacity and is extremely heavy. Where bonsai pots are concerned it simply takes up space that could be filled with a much better quality product that can actively benefit the tree concerned. Gravel is cheap but ultimately only of use for driveways or making concrete.

 

HEN GRIT / PIGEON GRIT
Birds need to ingest hard grit type materials to aid their digestion. Many types of bird grit are available as nice finely graded materials. They are normally mixtures of fine shingle, crushed stone including lime-stone, and crushed shells. Their make-up normally produces a high pH unsuitable for many plants. In tests on our nursery over 5 years these materials proved no more successful than horticultural grit-type materials and proved of very limited use.

 

CAT LITTER
Cat litter comes in various forms and it’s use in bonsai cultivation has become common over the last few years. Cat litters based on fired clay materials provide a perfect growing media where they are blended with other materials. Cat litters are cheap and readily available but one overriding concern needs to be considered.

Many cat litters contain deodorizers and disinfectants. Some leading brands have proven to be perfectly OK for growing plants, but bear in mind that cat litters are not designed for this use. In the event that a manufacturer or supermarket own brand changes the material or composition of the material within the bag, either to reduce cost or comply with some new health regulation, the results could prove catastrophic to your bonsai trees. We know of one U.K bonsai club that suffered massive tree losses due to this happening. Indeed, following a period of testing, we lost several trees worth a total of over £1k. You will not have recourse to claim against your retailer due to not using the product as instructed.

Another factor concerning cat litters is that of pH. This is of absolutely no concern to kitties taking a dump, but it can have profound effects on your treasured bonsai trees. Most cat litters we have tested prove to be either very acidic or very alkaline.
Our recommendation is to always use horticulture certified materials guaranteed to be fit for the purpose. The loss of just one tree will completely offset the savings you may make using cat litters.

 

SERAMIS
A very high quality fired clay product from Germany. Popular for years with hydroponics growers and commercial plant nurseries. Ultra light weight with 80% air filled porosity and a water absorption capacity of 100% it’s own weight. Has a neutral pH value. A superb product for mixing with other growing media or using right out of the bag. Seramis is frost proof and will last indefinitely. We would recommend using a good quality organic fertilizer like Green Dream Bonsai fertilizer when planting bonsai in inert inorganic materials like Seramis.

 

BARK PRODUCTS
There are dozens of bark-based products available for horticultural use that range from sublime to absolute trash (literally). Sorting out the good from the bad is not easy and may involve some careful experimentation. In general, branded products from specialist suppliers will be much better than own brand stuff from your local DIY warehouse type outlet. As with all things in life you get what you pay for so bear that in mind. Our advice is to buy from a specialist supplier with in-depth practical knowledge of the products use, particularly in relation to bonsai tree cultivation. Sadly, most bark producers will only supply in bulk so a specialist retailer needs to be consulted.

Different types of tree bark provide different growing environments for plants. Their moisture holding capacity and pH can vary dramatically. In general bark products have very low inherent nutrient content and tend towards being acidic in nature.

 

COMPOSTED BARK
Good quality composted bark is a fine fibrous and springy material that holds a lot of moisture and is very helpful in a bonsai soil mix to aid moisture retention in summer. However, in winter it can hold too much water, and may cause oxygen starvation leading to root rot so use it wisely!

Depending upon the source, composted bark can be a superb product for adding to blended bonsai soils. Some cheap products are little more than shredded composted garden waste from domestic waste streams with a little bark added. Other bark products may contain large volumes of wood and cambium that can make the product very wet and slimy. In fact products can vary so much that using composted bark can be a minefield. Top quality composted bark is just that, carefully graded bark from forestry operators and sawmills that is cleaned, graded and composted slowly over a long period prior to careful and accurate grading. We have never seen this on general sale in garden centres or DIY stores.
To see our top quality Composted Bark click here.


CHIPPED BARK
Chipped bark is available in dozens of formats from as many suppliers for a multitude of uses from surfacing children’s playgrounds to growing rare orchids.

Bonsai mixes need a top quality graded pine bark with less than 3% wood fiber. This will have a pH of around 5 and when mixed with other products should help to buffer the pH, especially if you have hard water.

Chipped bark should have very little fines (dust) and offers a high AFP (air filled porosity). It will have very little inherent nutrient and poor nutrient-holding capacity, but holds moisture well, provides a warm airy rhizosphere and a nice springy soil mix that will help to bring your bonsai through a hard freezing winter. Bark also seems very good at encouraging mycorrhiza to flourish. Top quality bark provides the perfect medium for propagation of cuttings.

The highest quality chipped bark products are made for orchid cultivation. Top quality orchid bark looks like you should pour milk over it and eat it for breakfast!
To see our top quality Chipped Bark click here.


BONSAI SOIL PRODUCTS EXCLUSIVE TO KAIZEN BONSAI


HORTICULTURAL PUMICE
Pumice is a solidified volcanic lava foam. Different forms of pumice are available in many areas of the world. There are as many different forms of pumice as there are places to find it. Pumice is used in a plethora of industrial applications from an abrasive to the aggregate in lightweight concrete.

For use in bonsai cultivation we need a soft, open-structured pumice, not a hard closed grain material like that used for lightweight concrete (that’s very commonly available). A lighter weight soft pumice will have a more open honeycomb structure that plant roots seem to adore. Having used pumice for 15 years we have found the perfect grade that we know works wonders with most commonly cultivated species used for bonsai. Pumice is also excellent for encouraging the growth of mycorrhiza.

Our horticultural pumice is light in weight, has a massive capacity to retain water and provides a fantastic AFP ratio. It’s frost-proof and will last indefinitely in your bonsai pots. In practice pumice produces best results when mixed with other bark and clay based growing mediums. Very fine grade pumice (1-3mm), should be avoided because it has a tendency to become waterlogged.
To see our Horticultural Pumice click here.

 

S-Te BONSAI
S-Te Bonsai is a completely new bonsai tree growing medium exclusive to Kaizen Bonsai. S-Te is a fired clay product with excellent moisture and nutrient holding capacity. Maintains an open structure soil that encourages dense fine root growth. Light weight, frost-proof and can be re-used over and over. Use in place of grit or crushed aggregates. S-Te has a neutral pH.
Formerly sold as Kyodama.
To see our S-Te Bonsai growing medium click here.

 

SUPALITE BLACK
This is an ultra light weight fired clay product with an amazing honeycomb structure. Mixed with other growing mediums Supalite Black holds soil open, promotes drainage, increases air capacity and dramatically reduces overall weight. Frost-proof, it resists breaking down for many years and can be used over and over. Just 465g weight per litre. Mix Supalite Black with other growing mediums such as bark or clay-based products to dramatically increase the AFP of soils and reduce weight. Neutral pH.
To see our Supalite Black Bonsai growing medium click here

 

LAPILLO
Lapillo is formed during violent volcanic eruptions and is the term used for small particles (2-65mm) that explode into the atmosphere and cool on their descent back to earth. This violent formation creates a fine honeycomb structure, which is perfect for cultivating plants. Our Lapillo has a very attractive maroon red through grey colour and is graded from 1-6mm in particle size, perfect for use in bonsai pots. The porosity of the material averages 50-60% by volume. Lapillo has high levels of iron, magnesium and other minerals vital to plant health.100% frost proof, Lapillo can be sieved out and reused many times. Has a similar structure to Japanese Fuji grit.
We recommend using Lapillo in conjunction with other bonsai soil products like Akadama, Bark and Moler. Mix up to 50% with other products to create a free draining, long lasting mineral-rich growing environment for your bonsai trees. The very attractive colour is also great for top dressing. No sieving. During processing and packaging Lapillo develops a fine dust that adheres to each particle. This need not be washed prior to use, simply mix Lapillo into your soil mix straight from the bag. After re-potting, water your tree thoroughly until the water draining from the pot is clear. A fantastic product, that is produced from natural deposits with a minimum of processing and transportation.
To see our Lapillo Bonsai growing medium click here

 

HORTICULTURAL MOLER
Known by many different brand names including Biosorb and Terramol Moler is a fired-clay product produced from naturally occurring montmorillonite that has formed from ancient algae deposits. It is calcined to produce a spectacularly porous light-weight granular material that is perfect for adding to plant growing mediums. Moler has an exceptional CEC and can hold it’s own weight in water. When carefully graded it provides a good AFP ratio. Frost-proof to at least -16 Celsius.

Moler is a quite acidic medium (pH5.5) and this should be borne in mind when working out the ideal soil mixture for your bonsai. We have found the use of 100% Moler to be less than satisfactory when compared to being mixed with other ingredients.

Moler changes colour quite dramatically when wet and this is very helpful to decide when watering is required.

Moler has a pale orange colour that is less than ideal but when mixed with other products this is of little consequence.

Moler is very cheap and light-weight and should be considered a very valuable component in most bonsai soils. It also makes a very good addition to orchid growing soils and is perfect for hydroponics.

Moler is available in a fine or coarse grade, which can be used straight from the bag.
To see our Horticultural Moler Bonsai growing medium click here

 

KAIZEN BONSAI PRE-MIXED BONSAI SOIL PRODUCTS
Graham Potter has spent more than 20 years cultivating bonsai in the U.K. Right from the first day he had an obsession with the relationship a plant shares with it’s soil. Keeping bonsai healthy requires a vibrant healthy root system above all else. In order to achieve this it’s necessary to use a growing medium suited to the plant and also to the pot involved. Soil requirements are also different depending upon the stage of development of a tree. For instance a different soil is required for bulking up young trees to that required for mature specimen bonsai.

Over the years Graham has studied horticulture professionally and has been experimenting relentlessly with different growing mediums. This has involved careful observation of thousands of bonsai trees, seedlings, cuttings and collected material. Having used every available product in a plethora of combinations with most plants cultivated as bonsai, Graham has a deep, broad and intimate understanding of exactly how things work.

Five years ago we began looking into the possibility of offering ready-to-use blended soils for bonsai that would take the guess work out of which soil to use in any given situation. Drawing on Graham’s extensive experience and our access to some excellent quality materials we began blending soil mixes and testing them in our nursery. Launched in 2010 our blended soils have been a massive success. Customer feedback has been universally positive.

Our blended soil mixes are offered on the basis of their moisture retention and drainage capacity with No1 being the most moisture retentive and No3 having the best drainage. A special mix is also available for Shohin bonsai that offer unique challenges with their diminutive pots.

Our soil mixes are carefully prepared in-house, with top-quality materials all sourced from U.K manufacturers. This ensures the very best quality control, less transportation costs, no import tax and so a cheaper price to our customers. Most of the cost of Japanese soils is transportation, handling and tax.

 

KAIZEN BONSAI BONSAI SOIL MIX No1
This fine particle mix is designed for trees that need lots of water in summer. Highly absorbent, No1 has the highest AFP of any fine-grained bonsai soil mix. It is designed with a very high CEC to keep bonsai trees healthy by getting the very best from the fertilizers you use.

Used straight from the bag No1 is the perfect mix for broad leaf trees that take up a lot of moisture in the summer and will help to increase the time required between watering. The fine grain particle size helps to alleviate surface evaporation and will also encourage the formation of a dense fine root structure. In winter very shallow bonsai pots can become a little too wet so we would recommend overhead protection from continuous rain.

Rich in minerals, but with little inherent nutrient, so you have control over the growth of your bonsai via the fertilizer you use. We highly recommend the use of Green Dream Bonsai Fertilizers.

Kaizen Bonsai No1 mix is particularly, but not exclusively, suited to….

Japanese maples
Trident maples
Beech
Ulmus species including Chinese elm
Satsuki azaleas
Erica species
Ligustrum (privet)
Vine species such as parthenosisus and wisteria
Mirtus species (myrtle)
Camellia and stewartia
Forsythia
Cotoneaster
Pyracantha

To see our No1 Bonsai growing medium click here

 

KAIZEN BONSAI BONSAI SOIL MIX No2
This medium particle mix is designed for trees that need lots of water in summer but also need lots of air in their soil. Highly absorbent No2 has the highest AFP of any medium grained bonsai soil mix. It is designed with a very high CEC to keep bonsai trees healthy by getting the very best from the fertilizers you use.

Used straight from the bag No2 is the perfect mix for broad leaf trees and conifers that take up a lot of moisture in the summer and will help to increase the time required between watering. The very high AFP ratio will encourage the formation of a dense fine root structure and very strong top growth in all species. As with all our soil mixes it is completely frost-proof so no winter protection is required. Rich in minerals but with little inherent nutrient so you have control over the growth of your bonsai via the fertilizer you use. We highly recommend the use of Green Dream Bonsai Fertilizers.

Kaizen Bonsai No2 mix is particularly, but not exclusively, suited to….

Japanese maples
Soft foliage junipers such as itoigawa, kyshu and shimpaku
Trident maples
English oak
Ulmus species including Chinese elm
Hawthorn species
Picea (spruce)
Satsuki azaleas
Erica species
Malus (crab apple)
Ligustrum (privet)
Carpinus (hornbeam)
Mirtus species (myrtle)
Camellia and stewartia
Larch
Prunus species
Pomegranate
Forsythia
Cotoneaster
Pyracantha

To see our No2 Bonsai growing medium click here

 

KAIZEN BONSAI BONSAI SOIL MIX No3
This bonsai growing media is absolutely unique and unlike anything you will have seen before. Coarse grain size from 3-8mm offers unprecedented capacity for growth and health. A no compromise product for those who demand the very best and expect nothing less than spectacular results. Be prepared to have everything you ever learned about soil challenged.

This soil mix is the ultimate growing media for a huge array of species used in bonsai. It’s massive AFP will generate roots like nothing you will have seen before. In spite of it’s coarse particle size it manages to hold a lot of water thanks to the fact that all it’s constituents are highly absorbent. The magic is in how that moisture is released back to the roots.
Absolutely frost-proof this mix will last for decades and can be sieved out and reused indefinitely. This is also one of the lightest weight bonsai soils available.

Rich in minerals, but with little inherent nutrient, so you have control over the growth of your bonsai via the fertilizer you use. We highly recommend the use of Green Dream Bonsai Fertilizers.

Kaizen Bonsai No3 mix is suitable for all species grown as bonsai but do bear in mind that moisture-loving plants and deciduous trees will need watering more often when placed in this mix.

No3 is particularly, but not exclusively, suited to….

Soft foliage junipers such as itoigawa, kyshu and shimpaku
Needle foliage junipers
All pine species
Trident maples
English oak
Hawthorn species
All Taxus species
Cupressus species
Olives
Cork oak
Berberis
Malus (crab apple)
Larch
Forsythia
Cotoneaster
Pyracantha
Ficus species
Succulents like portulacaria

To see our No3 Bonsai growing medium click here

 

KAIZEN BONSAI SHOHIN BONSAI SOIL MIX
As we have already discussed, small pots present us with quite a challenge in terms of growing and maintaining bonsai trees. These little trees have to get a great deal from a tiny amount of soil and so the soil needs to be an exceptional one. Getting the right ratio of water- holding, AFP and nutrient capacity is extremely difficult, especially considering we also need to insure the trees can go more than a couple of hours between watering in summer.

Our Shohin Bonsai Soil is as close to perfection for growing tiny trees as we think it’s possible to get. A remarkable range of intricately prepared materials are brought together in a truly unique and remarkable high-performance growing medium.
Suitable for all trees grown as shohin bonsai we are confident that this very special medium will set a standard for bonsai soil products and is guaranteed to be a revelation.

To see our Shohin Bonsai growing medium click here

 

 

GLOSSARY OF TERMS

AFP – Air filled porosity. A measure of the volume of air contained within a soil.

CEC – Cation exchange capacity. A measure of a soils ability to attract and hold plant nutrients and release them to plant’s roots.

Cryosuction – Describes the phenomena whereby water travels out of a soil particle towards an area of ice formation.

Gaseous exchange – The movement of air within a growing medium.

pH – The measure of a soils relative acidity/alkalinity. pH 7 is neutral, pH0 is extremely acid, and pH14 is extremely alkaline.

Respiration – The process of breaking down carbohydrates to release energy for growth.

Shohin – A Japanese classification for small trees up to 8” tall.

 

 

This article is published as a general guide to creating and using bonsai soils. We do not accept any liability whatsoever.
© Kaizen Bonsai Ltd. January 2011
If you would like to use all or parts of this guide please ask. info@kaizenbonsai.com